This invention relates to vehicle seats of the type in which a seat part (normally comprising a seat rest and a back rest) is supported and guided for upward and downward movement relative to a base part by means of a spring suspension. The suspension includes spring means capable of counterbalancing the load of the seat part and its occupant so that the seat part, under static conditions, is located at a mid-ride position substantially midway between UP-stop and DOWN-stop positions. Said spring means can comprise any means of taking up, storing and giving out energy so that the seat and its occupant can oscillate between the UP-stop and DOWN-stop positions when subjected to external forces.
Such a vehicle seat assists in isolating the occupant from vibrations transmitted from the vehicle wheels, particularly as the vehicle passes over rough terrain. However, the necessary freedom of the seat part to move up and down carries with it the danger, in the case of a driver's seat, of the driver's feet losing correct contact with the control pedals.
Previous attempts to overcome this difficulty have largely been based on the concept that the seat part of a suspension seat should move pivotally about a horizontal axis through the knee-hinges of the driver, since if such a knee-hinge axis (sometimes referred to herein as the K axis) is fixed relative to the base part of the seat, the lower leg, and hence the ankles, of the driver can remain stationary irrespective of the movements of the seat. However, if the seat rest and back rest of a seat part move in this way, the driver's back inclination will vary with rise and fall of the seat. This difficulty can be overcome by incorporating a linkage to change the inclination of the back rest with changes in the inclination of the seat rest. An example of a suspension seat designed in this way is found in British Patent Specification No. 1,156,942.
Such suspension seats have the disadvantage of a relatively large fore and aft movement of the entire part of the driver's body above his hip (H axis) since the knee hinge axis is the axis about which the seat rest and the hip region of his body must swing.
The present invention is based on the realisation that if a suspension seat can be designed to move in such a way that the seat occupant's lower leg will move about a horizontal transverse axis through the occupant's ankle hinges (A axis), the knee hinge axis accordingly being free to swing about the ankle hinge axis, such a seat will afford the driver a high degree of control over the vehicle over a wide range of seat movement. Moreover, it is found that a seat movable in this way affords a high degree of comfort to the driver.
Accordingly an object of the invention is to provide a suspension seat capable of moving the seat occupant's lower leg about a horizontal transverse axis through his ankle hinges, at the same time causing the seat occupant's back to move up and down with the back rest of the seat at a substantially constant inclination to the vertical. References herein to `the vertical` assume the vehicle to be on horizontal ground.
This object is achieved by the provision of a vehicle seat comprising a seat part, which includes a seat rest and a back rest, and a base part, the back rest being supported by a spring suspension which is effective to restrict movement of the back rest to a substantially vertical direction, the suspension including means connecting the seat rest to the back rest for movement about at least two horizontal transverse axes through instantaneous centres of rotation, said centres being the instantaneous centres of rotation of the thigh of a seat occupant relative to any two parts of the seat which move relative to each other during upward and downward movement of the seat part relative to the base part, the thigh of the seat occupant being movable longitudinally and pivotally in such a way that the ankle hinge axis of the seat occupant remains stationary relative to the base part. The reason why such a construction achieves the object of the invention will be explained later.
The present invention also provides a vehicle seat comprising a seat part having a seat frame mounted for upward and downward movement relative to a base part by means of a spring-biassed scissor-action suspension (as defined) the linkage arms of the suspension being arranged to effect upward and downward movement of the seat frame substantially without tilting movement, the seat part further having a seat pan for supporting a seat cushion, the seat pan having pivot connections to the seat suspension such that during upward and downward movement of the seat, movement of the rearward part of the seat pan takes place in an upward and downward direction such that the movement of the H axis is vertical, and the seat pan tilts to cause the K axis to move along an arc permitting the A axis to remain stationary, the H, K and A axes being as herein defined.
By the expression `scissor-action suspension` is meant a suspension which comprises a pair of rigid links having a common pivot connection in the manner of a pair of scissors, or a plurality of pairs of such links having a common pivot axis, one end of each link of the or each pair having a pivot connection to a seat part of the seat and the opposite end of each link of the pair having a pivot connection to the base part of the seat, the axes of the pivot connections all being parallel to one another, some of the pivot connections being fixed pivot connections but the other being sliding or rolling connections sufficient to give freedom of movement to the suspension in a substantially vertical direction only, an example being described in U.S. Pat. No. 3,109,621.